1. Field of Invention
The present invention relates to an optical switching element (light valve) used for optical communication and optical computation. The invention also relates to an optical switching element used in an optical storage device, an optical printer, an image display device, etc., and particularly in an image display device.
2. Description of Related Art
A conventional optical switching element comprises a liquid crystal. As shown in a schematic configuration of FIG. 16, a conventional optical switching element 900 comprises polarizers 901 and 908, glass plates 902 and 903, transparent electrodes 904 and 905, and a liquid crystal 906 or 907, so that when a voltage is applied between the transparent electrodes, the direction of liquid crystal molecules is changed to rotate the surface of polarization, resulting in optical switching. A conventional image display device comprises a liquid crystal panel having such optical switching elements (liquid crystal cells) arranged in a two-dimensional form, in which the direction of liquid-crystal molecules is controlled by adjusting the applied voltage to achieve grayscale.
However, a liquid crystal has low responsiveness, and is operated at a response speed of only several milliseconds. Therefore, it is difficult to apply an optical switching element comprising a liquid crystal to optical communication, optical computation, an optical memory device such as hologram memory, an optical printer, and the like. Also, an optical switching element comprising a liquid crystal is subject to the problem that the utilization efficiency of light deteriorates due to polarizers.
Higher image quality has recently been demanded in image display devices, and thus there has been a demand for an optical switching element which permits more precise display of grayscale than an optical switching element using a liquid crystal.
Accordingly, an object of the present invention is to provide an optical switching element which permits high speed response with low loss of light. Another object of the invention is to provide an image display device which can be relatively simply manufactured with high yield, and which permits color display with high resolution at low cost. A further object of the present invention is to provide an image display device which permits thinning of an optical system for illumination and which has precise grayscale characteristics.
In order to achieve the objects, the present invention provides an optical switching element comprising a light guide which permits extraction of light when a transmissive extraction surface is brought into contact with the light guide, and extraction of leaking evanescent light when the extraction surface is brought near the light guide at a distance of half a wavelength, preferably xc2xc wavelength, therefrom, whereby light can be turned on and off at high speed by finely moving a light extraction unit provided with the extraction surface for about a wavelength or less. Namely, in accordance with the optical switching element of the present invention, the light guide has a total reflection surface capable of transmitting light by total reflection, and the light extraction unit has the transmissive extraction surface which can be moved between a first position at not more than the extraction distance from the total reflection surface, where evanescent light leaks, and a second position at not less than the extraction distance therefrom. Since the optical switching element is capable of extracting evanescent light, it does not necessarily require adhesion (close contact) to the total reflection surface of the light guide, and exhibits high reliability. An off state is caused by movement from the first position to the second position at a distance of about a wavelength from the total reflection surface, and it is thus possible to provide an optical switching element which can be operated at high speed. Also, in the off state, light from the light guide is totally reflected without leaking, and thus an optical switching element having high contrast can be provided.
In the optical switching element, the light extracted by the extraction surface may be appropriately processed by an emission member so that the light can be output to the outside, thereby permitting on-off operations. For example, at least one of an emission surface and a reflection surface at a different angle from the total reflection surface may be provided on the emission member so that the light extracted by the light extraction unit can be output to the outside. As such an emission member, a micro prism or micro lens can be used for efficiently outputting the light extracted by the extraction surface to the outside. As the emission member, it is possible to use an optical element having the shape of a truncated cone or truncated pyramid which expands in the emission direction. The use of such an optical element enables a further increase in the efficiency of light extraction, and the direction of the extracted light to be close to the direction perpendicular to the total reflection surface of the light guide.
Also a light scattering body can be used as the emission member so that the extracted light can be emitted to the outside by scattering by the emission member. Further, as the emission member, a member which can emit light by extracted light, for example, a member which uses ultraviolet light as light to be transmitted to the light guide unit, and which contains a fluorescent agent emitting light when the ultraviolet light is applied thereto, can be used.
Further, an optical switching element can be provided in which wavelength selectivity is imparted to the light extraction unit to develop different colors. For this purpose, the extraction surface or the emission member may be provided to function as a color filter, a material having wavelength selectivity may be used as a scattering material or a light emitting agent.
The light extraction unit may be a transmissive type in which extracted light is guided to the side opposite to the extraction surface. Such a transmissive extraction unit is disposed on the light emission side with respect to the light guide unit to function as an optical switching element. On the other hand, the light extraction unit may be a reflective type in which extracted light is guided to the extraction surface side, and such a reflective light extraction unit is disposed on the side opposite to the light emission side with respect to the light guide unit to function as an optical switching element. Also the light extraction unit may be an emissive type which emits light by the light extracted by the extraction surface. In cases where the extracted light is scattered, or the light emitted by extracted light is emitted, a light absorbing layer is disposed on the side opposite to the emission side with respect to the light guide unit to absorb extraneous light, thereby improving contrast.
Further, a non-movable light processing unit may be provided on the emission side where the light extracted by the light extraction unit is emitted, and provided with a wavefront converting function, a wavelength selecting function or a light emitting or scattering function in place of the emission member having a scattering or wavelength selecting function and provided on the light extraction unit for switching operations. By providing such a light processing unit, it is possible to simplify the configuration of the light extraction unit, facilitate movement due to a decrease in the operation load, and enable high-speed switching operations.
In such an optical switching element, the light extraction unit may be separately supported so that it can be moved at a proper distance from the light guide. Where a thin film is laminated on the light guide with spacers therebetween, and the light extraction unit is supported by the thin film, it is possible to maintain an appropriate distance from the light guide, and unitize the light extraction unit. In the case of the transmissive light extraction unit, the light extraction unit can be supported by using a transmissive thin film laminated on the emission side through spacers. In the case of the reflective light extraction unit, the extracted light can be reflected by a reflective thin film laminated on the side opposite to the emission side through spacers. Also a reflective or non-transmissive thin film may be used for preventing extraneous light, thereby increasing a contrast.
In the optical switching element in which the light extraction unit is supported by a thin film, the light extraction unit supported by the thin film may be driven by using an appropriate driving mechanism to operate as a switching element. By providing a driving unit capable of driving directly or indirectly the light extraction unit supported by the thin film in the optical switching element, it is possible to provide the optical switching element with the driving mechanism integrated therein. As such a driving mechanism, an element having a piezoelectric conversion function, such as a piezo element or the like, may be used; however, the light extraction unit may be electrostatically driven by the driving unit so that electrostatic attraction or repulsion can be used, and a micro machine can be realized by using a thin film technique. This permits a thin type to be formed with a simple mechanism, and improves productivity. Therefore, a small optical switching element can be provided at low cost. For example, a transparent electrode can be provided on the total reflection surface, and an electrode opposite to this electrode can be provided so that the optical switching element can freely be controlled by applying a voltage thereto.
In the optical switching element in which the light extraction unit is supported by the thin film and electrostatically driven, the transparent electrode is preferably disposed on the light guide side with respect to the emission member for emitting light by optically processing the light extracted by the extraction surface. This can decrease the distance between the transparent electrode provided on the total reflection surface and the transparent electrode provided on the thin film side, and drive the optical switching element with a low driving voltage. When the thin film is made of silicon, electrostatic thermal junction to glass spacers can be made. The use of electrostatic thermal junction requires no adhesive layer, and thus further facilitates assembly in which the distance between the thin film and the total reflection surface is kept constant. Each of the spacers for defining the distance between the thin film and the total reflection surface has a prismatic form so as to support the light extraction unit at each of four positions around, thereby minimizing the arrangement space of the spacers, and securing the extraction surface having a wide area
Where a plurality of optical switching elements in accordance with the present invention are used and arranged in a two-dimensional form, and the light guide is connected thereto so that light can be transmitted, an image display device capable of two-dimensional display can be realized. The image display device has a high response speed and can be operated at high speed, and can provide an image with a high contrast. The image display device can also express grayscale in a time division manner by utilizing the high-speed properties of the switching elements, and thus obtain a high quality image. Since light intensity is not decreased by a polarizing filter or the like, and light is extracted directly from the light guide, the image display device capable of displaying a bright image can be provided.
In such an image display device, the light extraction units are supported by the thin film laminated on the light guide through spacers, thereby making the device thin and small. In order to electrostatically drive each of the optical switching elements arranged in a two-dimensional form, it is possible to use a scanning electrode and a sub-scanning electrode perpendicular to the scanning electrode. These electrodes are disposed on the light guide and the thin film or the light extraction units so that an image can be displayed by driving each of the optical switching elements arranged in a two-dimensional form.
Also color display can be realized by imparting wavelength selectivity to the light extraction unit. Further, a light source capable of supplying light of the primary colors to the light guide in a time division manner can be used for permitting color display by synchronously controlling the respective optical switching elements. Therefore, the image display device of the present invention has the effect of obtaining a high quality color image and enabling thinning of the device.